Headwaters,defined here as first-and secondorder streams,make up 70％-80％ of the total channel length of river networks.These small streams exert a critical influence on downstream portions of the river network by:retaining or transmitting sediment and nutrients;providing habitat and refuge for diverse aquatic and riparian organisms;creating migration corridors;and goving connectivity at the watershed-scale.The upstream-most extent of the channel network and the longitudinal continuity and lateral extent of headwaters can be difficult to delineate,however,and people are less likely to recognize the importance of headwaters relative to other portions of a river network.Consequently,headwaters commonly lack the legal protections accorded to other portions of a river network and are more likely to be significantly altered or completely obliterated by land use.